Dall's porpoise (scientific name: Phocoenoides dalli) is one of six species of marine mammal in the family Phocoenidae. The other five are the Finless porpoise, the Spectacled porpoise, the Harbour porpoise, the Gulf of California harbor porpoise, and Burmeister's porpoise.
|Dall's porpoise (Phocoenoides dalli) Source: NOAA National Marine Fisheries Service|
|Size comparison of an average human against Dall's porpoise. Source: Chris Huh|
Dall’s porpoises are highly acrobatic and are often seen swimming at high speed, darting to and fro, riding the bow waves of boats and engaging in slow rolls at the surface. Because they are black and white, boaters sometimes misidentify them as Killer whales.
They are small cetaceans, with robust bodies and small heads, flukes, and flippers. eproductively, they differ from many cetaceans by breeding annually, calving in June or July and mating soon thereafter. This means females are nursing young while pregnant with next year’s offspring. A calf often stays with its mother until the next one is born. Females favor certain areas of the ocean for calving; at calving time, most males, juveniles, and females without offspring stay farther south.
Dall's porpoises have very tiny teeth: each tooth is about the size of a grain of rice. They feed on a great variety of prey, from squid to deepwater fish to small schooling fish, which they swallow whole.
There is some dispute in the scientific community about whther there are two sub species:
- A dalli-type occurs throughout of the species’ range, from the west coast of North America to Japan.
- The truei-type, identified by a broad lateral white patch, which inhabits the western north Pacific, and migrates between wintering grounds off the Pacific coast of northern Japan and summer breeding grounds in the central Okhotsk Sea, and constitutes one population (IWC 2002).
Dall's porpoises are the largest of the phocoenids. They typically reach a length of 1.8 to 2.0 meters, rarely more than 2.2 meters. At birth, the length is between 0.85 and 1.0 meters.
Body mass in adults varies from 130 to 220 kilograms. There is a sexual dimorphism, with males being larger than females.The body is stocky and more powerful than other members of Phocoenidae. The head is small and lacks a beak although there is a sloping forehead. The flippers are small, pointed, and located near the head. The dorsal fin is triangular in shape with a hooked tip.
There are three color patterns observed in the Dall's porpoises. The first is a uniform black or white throughout the entire body. The second pattern consists of intermixed stripes of black and white running along the length of the body. Finally, there is the most common color pattern observed, that of P. dalli dalli. This is defined as having a dorsal area uniformly black with a white ventral side. The white ventral patch begins far behind the flippers. The dorsal fin, flippers, and fluke are black with some white at the tips. The color pattern of P. dalli truei is different only in the distribution of the white ventral patch. The white patch begins ahead of the flippers rather than far behind them, and P. dalli truei is often longer and slimmer than P. dalli dalli. (Genther, 2000; Klinowska, 1991; Reeves and Leatherwood, 1994)
Other Physical Features: Endothermic; Homoiothermic; Bilateral symmetry; Polymorphic
Dall's porpoises usually occur in small groups of 10 to 20 individuals, although aggregations of at least 200 have been reported. They occur only rarely in groups of mixed species, although further north they are sometimes seen in the company of harbor porpoises, especially in the deep waters off Alaska and in Prince William Sound. They have also been spotted with gray whales. Migration north in summer and south in winter has been reported.
Dall's porpoises do not exhibit the typical shy and secretive behavior of most other porpoises. They are frequently seen charging boats and bow-riding. They are the fastest of the Phocoenidae and reach speeds of up to 35 mph. They frequently swim in a zig-zag pattern with fast, jerky, steep angled turns. Dall's porpoises may surface with a slow roll, a fast roll, or a rooster-tailing roll. The rooster-tailing roll is often used to identify the species in the wild. The spray resulting from this roll is a cone of water coming off the head of the porpoise which looks like a "rooster tail" due to the quick speed and steep angles at which the species surfaces. (Klinowska, 1991)
Key Behaviors: natatorial; motile; migratory; social
Voice and Sound Production
Communication channels used by this species includes tactile and acoustic. As in most Phocoenidae, Dall's porpoises use a form of echolocation to navigate, capture prey, and perhaps to communicate with conspecifics. They also use a variety of audible clicks and whistles. They also utilise touch for social communication.
Maximum longevity in the wild is approximately 22 years. Observations: Average longevity is around 16-17 years (Margaret Klinowska 1991).
Little is known about the reproductive biology of Dall's porpoises. Two calving periods have been reported for portions of the eastern North Pacific, one in winter, from February through March, and the other in summer, from July through August.
Some segregation of animals seems to occur with juveniles found closer to shore and larger adults well offshore. In offshore areas, females in late pregnancy or lactation seem to be distributed in northern areas, and southern areas are mainly occupied by males and females not accompanied with calves. This seems to indicate that not all females become pregnant every year.
Females usually reach sexual maturity between the age of three to six years, whereas males reach sexual maturity between the ages of five to eight years. Gestation is believed to last about 11 months, and lactation periods are usually about two years.
Phocoenoides dalli dalli appear to have three major breeding grounds. Two occur in the North Pacific north of 45 degree latitude, and another breeding site occurs in the central Bering Sea. Phocoenoides dalli truei may breed off the northern coast of Japan. (Klinowska, 1991; Reeves and Leatherwood, 1994)
Individual females probably do not breed every year. Breeding intervals may be as long as three to four years because of the length of dependence of calves. Mating is likely to occur after the calving seasons each year which occur in winter, from February to March, and in summer, from July to August. Females feed and care for their offspring for extended periods of time. It is likely that some form of extended learning occurs during this period as well. Male parents do not contribute parental care.
Key Reproductive Features: Iteroparous; Seasonal breeding; Gonochoric/gonochoristic/dioecious (sexes separate); Sexual; Viviparous
Distribution and Movements
Dall's porpoises, Phocoenoides dalli, are cool water porpoises inhabiting the North Pacific Ocean and adjacent seas. The central Bering Sea marks the northern boundary of their range and, although they prefer colder water, Dall's porpoises are found in the warmer waters of Baja California on the east to southern Japan on the west. They are frequently observed in these lower latitudes during the winter months.
There are potentially two subspecies of Dall's porpoises, although they may simply be color morphs, P. dalli dalli and P. dalli truei. Phocoenoides dalli truei is abundant only off the Pacific coast of northern Japan. (Genther, 2000; Klinowska, 1991; Reeves and Leatherwood, 1994)
Generally the colder waters of the North Pacific are home to Dall's porpoises. They are observed inshore and offshore (coastal and pelagic). They are a deep water species, so when they approach the coast they usually follow canyons or deep channels. They are also commonly observed in sounds and inland passages where these meet the open sea. (Klinowska, 1991; Reeves and Leatherwood, 1994)
Habitat Regions: Temperate; Polar; Saltwater or marine
Dall's porpoises apparently feed at night and depend to some degree on the deep scattering layer, that is the fauna which travels upwards each night from the deeper parts of the ocean's water column. Food species as determined from stomach contents include squid and other cephalopods, lanternfish, Pacific hake, jack mackerel, herring, sardines, mollusks and crustaceans. Dall's porpoises are thought to be capable of deep diving because mesopelagic, bathypelagic, and deep-water benthic species are represented in the diet. (Klinowska, 1991; Reeves and Leatherwood, 1994)
Dall's porpoises are important predators of fish and cephalopods in the ecosystems in which they live. Killer whales and sharks are believed to be the primary natural predators of Dall's porpoises. They largely escape predation through their large body size, agility in the water, and their habit of traveling in groups. Their coloration may make them difficult to see in the water as well.
Economic Importance for Humans
The only direct commercial harvest of Dall's porpoises is a traditional coastal harpoon fishery in Japan which accounts for annual harvests of about 6000 animals to compensate for the perceived shortage of whale meat. Dall's porpoises contribute to marine ecotourism through their gregariousness and their aquatic antics. (Reeves and Leatherwood, 1994) Dall's porpoises have no negative effects on humans.
Threats and Conservation Status
The IUCN Red List notes that the species "is widespread and abundant, with current range-wide population estimates of more than one million animals. The species was killed in high-seas driftnet fisheries operations during the 1970s and 1980s, but these fisheries have now been banned, and by-catch levels were not considered sufficiently high to cause population declines. While incidental and directed takes in Japanese coastal waters as well as incidental takes in Russian waters are ongoing (with combined removals on the order of 20,000 annually), neither threat is likely to have caused a range-wide decline sufficient to warrant listing in a category of threat. "
Dall's porpoises are not directly exploited in the eastern Pacific, but serious conservation problems are centered in the western Pacific where, during the 1980's, Dall's porpoises were intensely hunted. Estimates suggested 40,367 Dall's porpoises were killed in 1989 from the Japanese hand-harpoon fishery alone. In recent years these numbers have declined because of the Japanese government's effort to regulate the hand-harpooning of these animals. In 1992 11,403 were killed. This species is often killed accidentally in the Japanese seas and off of North America by drift nets set for salmon. It has been estimated that up to 20,000 porpoises are entangled and drowned in these nets off of Japan and up to about 4100 off of North America annually. Due to international negotiations between Japan and the United States, along with new fishing gear and techniques, the incidental take has been reduced drastically. However, the conservation of Dall's porpoises remains a major issue. (Klinowska, 1991; Reeves and Leatherwood, 1994)
IUCN Red List: Lower Risk - Conservation Dependent
US Federal List: No special status
CITES: Appendix II
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